3 - Effects of UV-B radiation (280–320 nm) on foliar saprotrophs and pathogens

Summary

Ozone (O₃), ultra-violet (UV) radiation and climate change

The earth's atmosphere contains about 3 nl l^-1 O₃, that fraction being continuously turned over. Approximately 10% is dispersed in the troposphere, which extends 15 km above the ground, and 90% in the stratosphere, which extends up to 50 km. While man's activities are causing some increase in tropospheric O₃, mainly around major urban areas, they are depleting stratospheric O₃ on a global scale (Anon., 1993a,b). Depletion occurs because of the release of chlorine-containing compounds, such as chlorofluorocarbons and carbon tetrachloride, that promote O₃ breakdown.

Solar radiation includes wavelengths as short as 200 nm but that below approximately 290 nm is absorbed in the atmosphere, mainly by O₃. Since energy per quantum of radiation increases as wavelength decreases, O₃ protects organisms from the most energy-rich, and potentially most damaging, wavelengths in solar radiation. The efficiency with which ozone absorbs UV decreases as wavelength increases, so progressive thinning of the O₃ layer will allow shorter wavelengths to reach the earth's surface as well as allowing higher fluxes to be transmitted of the wavelengths already penetrating. It is significant that the cut-off wavelength is not constant but varies with season and time of day. For example, in Reading, England (51.5° N), the shortest detectable wavelength varied from 302 nm in January to 294 nm in July, and from 294 nm at noon to 300 nm at 17.00 hours in July (Anon., 1993b).